The present invention generally relates to methods for processing nickel-base superalloys. More particularly, this invention relates to a method for producing an article from a nickel-base superalloy, in which nonuniform nucleation tendencies are minimized and grain growth is controlled in the alloy during supersolvus heat treatment, so as to yield an article characterized by a microstructure with a desirable, substantially uniform grain size distribution.
Powder metal gamma prime (γ′) precipitation-strengthened nickel-base superalloys are capable of providing a good balance of creep, tensile, and fatigue crack growth properties to meet the performance requirements of certain gas turbine engine components, such as turbine disks. Typically, components produced from powder metal gamma-prime precipitation-strengthened nickel-base superalloys are consolidated, such as by hot isostatic pressing (HIP) and/or extrusion consolidation. The resulting billet is then isothermally forged at temperatures slightly below the gamma-prime solvus temperature of the alloy to approach superplastic forming conditions, which allows the filling of the die cavity through the accumulation of high geometric strains without the accumulation of significant metallurgical strains. These processing steps are designed to retain a fine grain size within the material (for example, ASTM 10 to 13 or finer), achieve high plasticity to fill near net shape forging dies, avoid fracture during forging, and maintain relatively low forging and die stresses. (Reference throughout to ASTM grain sizes is in accordance with the scale established in ASTM Standard E 112.) In order to improve fatigue crack growth resistance and mechanical properties at elevated temperatures, these alloys are then heat treated above their gamma-prime solvus temperature (generally referred to as supersolvus heat treatment), to cause significant, uniform coarsening of the grains.
During conventional manufacturing procedures involving hot forging operations, a wide range of local strains and strain rates may be introduced into the material that can cause non-uniform critical grain growth during post forging supersolvus heat treatment. Critical grain growth (CGG) as used herein refers to random localized excessive grain growth in an alloy that results in the formation of grains whose diameters exceed a desired grain size range for an article formed from the alloy. Critical grain growth may be manifested as individual grains that exceed the desired grain size range, multiple individual grains that exceed the desired grain size range in a small region of the article, or large areas of adjacent grains that exceed the desired grain size range. Because critical grain growth is believed to be driven by excessive stored energy within the worked article, the grain diameters of these grains are often substantially larger than the desired grain size. In view of the above, the term “uniform” will be used in reference to grain size and growth characterized by the substantial absence of critical grain growth. Desired ranges for forged gas turbine engine components often entail grain sizes of about ASTM 9 and coarser, such as ASTM 3 to 9, but are generally limited to a range of several ASTM units in order to be considered uniform, such as ASTM 6 to 8.
The presence of grains within a component that significantly exceed the desired grain size range are highly undesirable, in that the presence of such grains can significantly reduce the low cycle fatigue resistance of the article and can have a negative impact on other mechanical properties of the article, such as tensile and fatigue strength. In addition to the case of critical grain growth described above, where the regions of critical grain growth can exhibit grain sizes substantially larger than the desired grain size range and a grain distribution that is therefore not uniform, components can also be produced with structures that are more uniform but still undesirable if the average grain size is slightly coarser than the desired grain size. As an example, if the desired grain size range for a nickel-base superalloy article is ASTM 6 to ASTM 8, random grain growth that produces individual or small regions of grains coarser than about ASTM 3, or large regions of the forging that are uniform in grain size but with a grain size coarser than the ASTM 6-8 range, will often be undesirable. Disks and other critical gas turbine engine components forged from billets produced by powder metallurgy (P/M) and extrusion consolidation generally exhibit a lesser propensity for critical grain growth than if forged from billets produced by conventional cast and wrought processing or spraycast forming techniques. However, such components are still susceptible to critical grain growth during supersolvus heat treatment.
Commonly-assigned U.S. Pat. No. 4,957,567 to Krueger et al. teaches a process for eliminating critical grain growth in fine grain nickel-base superalloy components by controlling the localized strain rates experienced during the hot forging operations. Krueger et al. teach that local strain rates must generally remain below a critical value, {dot over (ε)}c, in order to avoid detrimental critical grain growth during subsequent supersolvus heat treatment. Strain rate is defined as the instantaneous rate of change of geometric strain with time. Further improvements in the control of final grain size have been achieved with the teachings of commonly-assigned U.S. Pat. No. 5,529,643 to Yoon et al., which places an upper limit on the maximum strain rate gradient during forging, and U.S. Pat. No. 5,584,947 to Raymond et al., which teaches the importance of a maximum strain rate and chemistry control. Implementation of the teachings of Krueger et al., Yoon et al., and Raymond et al. has generally required the use of very slow ram speed control of the forging press head (generally with a simple linear decay vs. stroke control scheme), coupled by simulative modeling to translate the press head deformation rate into actual metal strain rate as a function of temperature, constitutive property data for the forging stock, die shape, and die or mult lubrication. While the teachings of Krueger et al., Yoon et al., and Raymond et al. have been largely effective in controlling critical grain growth, mechanical properties would further benefit from improved control of the grain size distribution in components forged from fine grain nickel-base superalloys, including a grain size distribution that is without critical grain growth and with the average grain size as fine as possible and as narrow as possible. Such a capability would be particularly beneficial for higher temperature, higher gamma-prime content (e.g., about 50 volume percent and above) superalloys that have been developed, such as René104 (R104) disclosed in commonly-assigned U.S. Pat. No. 6,521,175 to Mourer et al., for which the degree of process control to achieve uniform grain size within the desired 6-8 range has been found to be more difficult.